The present invention relates to a device for the positive modulated braking of the thread for weft feeders of shuttle-less looms, particularly gripper looms, projectile looms, and air-jet looms.
In the present description, the term "positive" relates to the modulated braking of the thread produced by the action applied to the braking means by an electric excitation or pick-up current that is modulated so as to match the variations in the mechanical tension of the thread during the weaving process.
More specifically, the present invention relates to a device for the positive modulated braking of the thread, that comprises a braking means having a continuous circular shape, typically a frustum-shaped body, which is supported frontally and coaxially to the drum of the feeder in order to engage, either directly or with the interposition of resilient elements, the thread that unwinds from said drum, and in which the frustum-shaped braking body is subjected to the action of an electromechanical actuation means capable of varying the pressure with which said body makes contact with the drum of the feeder, said actuation means being supplied with said modulated excitation current.
In conventional devices for positive modulated braking the braking body is subjected to the electrodynamic action produced by the interaction of the excitation current that flows in a moving coil associated with the braking body and the magnetic field of a permanent magnet that is fixed with respect to said braking body.
A device of the above-mentioned type, hereinafter referred to as a "conventional device", is disclosed for example in published European patent application No. 536,088.
Although both of these conventional devices achieve effective modulated braking, they have structural and functional drawbacks. From a structural point of view, they are relatively complicated and bulky and require accurate manufacture of the moving coils and of the fixed permanent magnet, between which a minimal air gap must be provided in order to achieve a significant electrodynamic action with low excitation currents. On the other hand, the conductor of the moving coils must be small in size and the coils must be formed by a minimal number of turns in order to avoid excessive increases in the mass and therefore in the inertia of the frustum-shaped body that supports said coil; this is the main structural drawback, since it in any case limits the maximum allowable value of the excitation current; accordingly, the electrodynamic action affecting the frustum-shaped body, which depends on the ampere-turns, is in any case limited to small values that are sufficient to produce the modulated thread braking action if the frustum-shaped body acts by direct contact on the drum of the device but are substantially insufficient if said body acts on the thread with the interposition of flexible elements, such as rows of flexible laminae or rings of bristles.